Tacrolimus is an immunosuppressive drug whose main use is after allogeneic organ transplant to reduce the activity of the patient's immune system and so lower the risk of organ rejection. It reduces T-cell and interleukin-2 (IL-2) activity. It is also used in a topical preparation in the treatment of severe atopic dermatitis (eczema), severe refractory uveitis after bone marrow transplants, and the skin condition vitiligo. It is a 23-membered macrolide lactone discovered in 1984 from the fermentation broth of a Japanese soil sample that contained the bacteria Streptomyces tsukubaensis.